Conventional single core autotransformers (also known as transmission line transformers) are well known in the electronics industry for impedance or voltage step-up and step-down applications. Autotransformers are known to work fairly well up to 1 GHz with an impedance ratio of 4:1 or less. At higher impedance ratios of greater than 4:1, however, the high frequency response is limited by the length of the windings as they approach a quarter wavelength and thereby induce a resonance. More specifically, the higher ratio impedance transformer's performance is limited by the electrical length of the secondary windings.
Broadhead, Jr. U.S. Pat. No. 3,244,998 discloses an impedance transformer circuit using a single toroid core that is suitable for operating up to about 80 MHz.
Wandel U.S. Pat. No. 5,216,393 discloses an impedance transformer circuit in which a double aperture ferrite core is used as well as an autotransformer having the first and second windings twisted together and wrapped through the apertures of the double ferrite aperture core. This patent discloses performance within a band width from 47 MHz up through 860 MHz.